The present invention relates to an image processing apparatus for processing a color image read out from a document on an image processing apparatus such as a digital type color copier for copying a color image and to an image forming apparatus for forming a color image with the use of this image processing apparatus.
Generally, various documents are used as an printing medium and there are sometimes the situations in which it is not necessary to faithfully copy a document in accordance with the use to which a copied image is put. In the case of a document, such as a newspaper and notebook, having a higher background tone, if such a background is faithfully reproduced, then characters in the background becomes lower in contrast and very illegible.
If the paper sheet is thinner in the magazine, etc., an image of a back page is often slightly visible on a front page side and, if an image is copied from the magazine page, then it is copied together with the image on the back page, so that a xe2x80x9cback pagexe2x80x9d emergence occurs.
In the case where a copy is made from the newspaper, notebook, magazine and so on, copying is made with a thinner density tone set by a manual adjustment, so that the background tone and the xe2x80x9cback pagexe2x80x9d emergence are not prominent.
The setting of the density as set out above is sometimes done automatically. In JPN PAT APPLN KOKAI PUBLICATION NO. 3-88569, the density adjustment level is automatically set by detecting the density distribution characteristic of an image as a whole and setting the density conversion characteristic with the density distribution characteristic as a parameter. By doing so, it is not necessary for the user to set the density level while considering the background tone of each document page. And copying is simply carried out.
In the case where the document, such as the newspaper, having the background tone is copied with the copying density adjusted to a low density level, the background tone becomes thinner in level but, at the same time, the characters also become thinner in density level and does not necessarily become legible and does not always look clean and clear. Further, the same thing is also true of a document whose back page is somewhat visible from the front page side.
Further, the same situation is again encountered in the case where the setting of the density level is automatically done as set out above. Indeed, it is not necessary for the user in the automatic setting method to set the density level with the background tone in mind and a readier copying operation can be carried out, but not only the background tone but also characters in the background becomes thinner.
In the case where color copying is carried out, another problem occurs. A color document is usually often given a background color tone intentionally and it is not always desirable to eliminate the background color. A distinction should be made between a document, such as the newspaper, whose background tone should be eliminated and a printed color document whose background tone should not be eliminated. For the former case only, the background should be eliminated.
For the document including a photograph, if the elimination of the background tone and that of a xe2x80x9cback imagexe2x80x9d emergence is effected all at a time over a whole image, then the photograph section needs to have its density level to be faithfully reproduced but the density level of the photograph will be lowered. That is, it is necessary to prevent a background tone from being eliminated, and a xe2x80x9cback imagexe2x80x9d emergence from being so processed, in the photograph section.
In the above-mentioned JPN PAT APPLN KOKAI NO. 3-88569, a xe2x80x9ccharacterxe2x80x9d section and xe2x80x9cphotographxe2x80x9d section are identified on a pixel-by-pixel fashion and a corresponding density-level conversion is done with their suitable density-level conversion characteristic. By this method it is possible to properly effect the density-level conversion of the xe2x80x9cphotographxe2x80x9d section. At the xe2x80x9ccharacterxe2x80x9d section included in the background, the background color tone will always be eliminated even if a given document is not desirable to have its background tone eliminated.
In the case where a color document, being prominent in a xe2x80x9cback imagexe2x80x9d emergence in particular, has its emergence level lowered by lowering its background tone, some effect is gained if it has not any background color tone. If, on the other hand, the color document having a background color tone is copied, a aback image, emergence is lowered but the background color tone will change, thus presenting a problem. Therefore, if this is the case, then a countermeasure is to eliminate the xe2x80x9cback imagexe2x80x9d emergence while reserving the background color tone.
Further, in the case of a color document having a background color tone in particular and having a greater background area over a document whole surface, uneven shades of density become prominent with the recording characteristics of an ordinary recording apparatus. If, in a digital copier for example, the background density is output at a constant level by detecting the background area, it is possible to suppress even the uneven shades of density.
An object of the present invention is to provide an image processing apparatus which, in the case of copying a document of a background density, lightens a background density and reserves a character density and, in the case of copying a document involving a xe2x80x9cback imagexe2x80x9d emergence, lightens the background image and reserves a xe2x80x9csurfacexe2x80x9d image density.
Another object of the present invention is to provide an image processing apparatus and image forming apparatus which, even in the case of a document additionally containing a photograph, converts a background density of a character area to another value and faithfully reserves the density of a photograph section and involves no change in color and in density.
Still another object of the present invention is to provide an image processing apparatus and image forming apparatus which, even if a document has a background color tone, can suppress a xe2x80x9cback imagexe2x80x9d emergence while reserving the background color tone and reduce an uneven shade of background density.
In order to achieve the above-mentioned object, the image processing apparatus of the present invention comprises density distribution calculation means for calculating a density distribution of a document image on the basis of input document image density data, density range calculation means for calculating a density range corresponding to a background density of the document image on the basis of the density distribution calculated by the density distribution calculating means, and conversion means for converting the document image density contained in the background density range calculated by the density range calculating means to another density value and outputting it.
The density distribution calculation means has a histogram preparing means for preparing a density histogram representing color features of the document on the basis of the input image data, and the density range calculation means has means for deciding, as a background density level of the document, a density having a greatest frequency in a low density area of the histogram prepared by the histogram preparing means and calculating the background density range on the basis of the background density level.
The conversion means has means for converting, to a value xe2x80x9c0xe2x80x9d, input image data below the background density level calculated by the density range calculation means.
The conversion means also has means for converting, to a value xe2x80x9c0xe2x80x9d, input image data below the lower background density level calculated by the density range calculation means and converting input image data greater than the background density level in accordance with a predetermined function.
Further, the conversion means has means for converting, to a predetermined value, the input image data below the background density level calculated by the density range calculation means.
Still further, the conversion means has means for converting, to a predetermined value, only input image data contained in a predetermined density range containing the background density level calculated by the density range calculation means and outputting the other input image data directly.
Further, the density range calculation means has means for deciding, as being the background density range, a density range near the background density level having a frequency down to a frequency smaller by a predetermined value than a frequency of the background density level relative to the image data.
The apparatus according to the present invention is characterized by further comprising means for setting the document as being a color document or monochrome document and that the conversion means effects first background density conversion with respect to the document set as being a color document and effects second background density conversion set as being a monochrome document, the second background density conversion differing from the first background density conversion.